The present invention relates generally to magnetic resonance (MR) imaging, and more specifically, to a system and method for transmitting multiple radio frequency (RF) channels via a multi-element RF coil assembly. In some embodiments, the number of independent RF channels communicated to the RF coil assembly may be less than the number of coil elements in the assembly. In such a case, one or more of the RF channels may be split and/or phase shifted for application to more than one coil element.
MR imaging in general is based upon the principle of nuclear magnetic resonance. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field, such as a B1 excitation field, which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One goal in MR imaging is to produce a homogenous B1 excitation field such that a desired magnetization effect caused by the B1 field will be produced as intended. In classic “birdcage” coils, the energy in an ideal B1 field, measured in Joules, is determined for the loops of a coil based upon:JN=J0·cos(ωt+NΔΦ)  Eqn. 1where ω is the characteristic Larmor frequency for the spins of interest, N is the number of loops in the birdcage coil, and ΔΦ is the angular distance between loops. Since all loops are electrically interconnected, the birdcage coil acts like a transmission line with one wavelength existing about the entirety of the structure. In normal 1.5 T imaging, the Larmor frequency for protons is about 64 MHz and permittivity of human tissue (or other objects of interest) is generally not a significant factor in producing appreciable B1 inhomogeneities.
However, in high field imaging, such as where the composite B fields are on the order of 3 T or 7 T, the Larmor frequency for protons is higher, due to f=γ·B. For example, in 3 T imaging, the Larmor frequency for protons is about 127 MHz. Thus, wavelengths for the RF transmissions become shorter and the permittivity of a tissue to be imaged can become a factor. The relative permittivity of human tissue can have values of ER from about 6 to 70. Significant phase changes as well as signal attenuations can occur in an RF transmission as it passes into and through an imaging tissue under these conditions. Such phenomena can cause both constructive and destructive interference with the RF transmissions from the other loops of an RF coil. Therefore, even when the transmissions from loops of a coil are carefully tuned to produce an ideal homogenous B1 field, inhomogeneities may still be produced at high Tesla fields and Eqn. 1 may not hold in reality. Inhomogeneous B1 fields lead to inaccurate flip angle distributions in a field of view (FOV) and dark areas in images.
One way to help prevent these inhomogeneities is to utilize a technique known as RF shimming. RF shimming involves adjusting the signal inputs for each loop of a coil assembly to account for expected or measured field inhomogeneities. “Passive” RF shimming includes splitting, phase shifting, amplifying, attenuating, or otherwise tuning the same RF waveform to produce varying inputs for each coil. “Dynamic” RF shimming includes producing unique RF waveforms for each coil and accounting for inhomogeneities in the design of the waveforms.
Classic birdcage coil assemblies are not known to implement RF shimming techniques as effectively as transverse electro-magnetic (TEM) coil assemblies. TEM coil assemblies have individual coil elements which can be driven separately, making them more ideal for multi-channel transmissions such as parallel transmission. However, known TEM coil assemblies generally experience coupling or mutual inductance between neighboring coil elements and non-neighboring coil elements, and between coil elements and the RF shield. This coupling is relied upon to characterize the TEM coil assembly as a single resonator in design techniques. Additionally, known TEM coil assemblies use either one RF waveform input, or a multi-channel RF input which is tailored to the structure of the TEM coils.
It would therefore be desirable to have a system and method for effectively reducing or eliminating B1 inhomogeneities in high field imaging. It would be further desirable for such system and method to present an RF coil assembly which is capable of operating without coupling and which is capable of transmitting a variety of multi-channel RF inputs.